1. Field of the Invention
The present invention relates to an optical signal processing circuit and a method for producing the same. In particular, it relates to an optical signal processing circuit and a method for producing the same, which are effective in applications for an optical multiplexer for optical communications and an optical signal processing apparatus that controls a super high-speed optical signal waveform.
2. Description of the Related Art
There is an optical signal processing circuit disclosed in Japanese publication of unexamined patent application No. (hereinafter referred to as JP-A-) 2000-098150 as one of the prior arts pertaining to optical signal processing circuits having a concave, flat or convex reflecting structure formed therein.
In the prior art, it has been proposed that optical coupling between an arrayed waveguide and an input/output waveguide is coupled by means of a reflecting structure.
However, the prior art does not provide any detailed method for producing the reflecting structure, and problems remain in the reflecting structure and in arrangement of the input/output waveguides.
FIG. 42 shows a configuration of a prior art slab waveguide lens function circuit.
The slab waveguide type lens function circuit 200 is structured so that arrayed waveguides 201a and 201b consisting of a single waveguide or a number of waveguides are connected to each other via a slab waveguide 202. Ends 201ae and 201be of respective waveguides are disposed on arcs centering around both ends P and Q of optical axes PQ of the slab waveguide 202. The configuration becomes approximately equivalent to a case where the ends 201ae and 201be of the waveguides are placed at positions of the front focusing plane and the rear focusing plane in a lens because both ends P and Q of the slab waveguide 202 are, respectively, made into the relationship of spatial Fourier transformation.
However, in the configuration, it is impossible that an image-formation of each other is established between two waveguides 201 and 201b as in an optical system using lenses and mirrors, and since it is necessary that the ends 201ae and 201be of the waveguides are disposed on arcs, the degree of freedom in creating circuits is low.
Further, no publicly known technique is provided in regard to a general configurational method of an optical signal processing circuit in which a reflecting structure of a concave, flat or convex plane or a periodic grooved structure is formed in an optical waveguide circuit to constitute a reflective optical system.
There is a one-dimensional optical waveguide as a technique that can be regarded as its particular case, wherein a reflecting structure of a distributed feedback type semiconductor laser and a distributed reflective type semiconductor laser, and an optical fiber diffraction grating are already known.
In actuality, however, these publicly known techniques are arts pertaining to a reflecting structure and diffractive structure, which are constituted in a single optical waveguide, and are not necessarily sufficient for circuit design of a two-dimensional planar waveguide having a concave, flat or convex reflecting structure or a periodic grooved structure.
Also, in an ion etching method (reactive ion etching method) that has generally and conventionally been utilized as a production technique for the above-described structure, since it is not possible to form smooth grooves perpendicularly and sufficiently deep, the method is not suitable for production of the above-described optical signal processing circuit.
The convergence ion beam etching method as has been disclosed in JP-A-H7-7229 can be considered as another groove formation art.
However, although the perpendicularity and smoothness of the grooved structure formed by this method are comparatively satisfactory, the convergence ion beam etching method has a slow etching speed, wherein it takes two or three hours to form a grooved structure of several tens of microns. In addition, the ion beam diameter is usually 5 through 10 microns in the convergence ion beam etching method, wherein further minute processing is difficult to perform. Therefore, concurrent use of a specified mask is taken into consideration. However, since the etching selectivity is slight between substances with respect to the ion beam, it is necessary to provide a thick resist film in view of forming deep grooves, and resultantly it is difficult to process minute patterns of 3 microns or less.
FIG. 43 shows a configuration of an optical nonlinear element having a pseudo phase matching section by a prior art periodic polarization inverting structure 502. In the prior art element, efficiencies of wavelength conversion and secondary higher harmonics generation, etc., are low if applied for practical uses, and the prior art element is driven only by a laser light having high intensity. Since the efficiencies are proportional to the square of an element length, a large-sized substrate may be used. However, it is extremely difficult to produce a large-sized substrate, and production cost thereof is expensive. Also, the application area of a large-sized element is narrowed in view of practical applications.
In the prior art laser processing, there is a limit in minute processing of a high wavelength, and the prior art laser processing is not suitable for a sub-micron process. This is a physical limit resulting from a diffraction limit of light. Utilization of a mask by electron beam tracing is taken into consideration. However, since light exponentially attenuates if the light transmits a window whose dimension is less than the wavelength, deep processing becomes impossible.
Conventionally, no configuration method of TE/TM mode coupler/splitter circuit that can be integrated in a waveguide has publicly been known.
In the prior arts, a specified pattern is formed by an electron beam exposure and tracing method, a structure is produced by dry etching, etc., and a diffraction grating is produced in a waveguide. (For example, DFB laser utilizing a chemical compound semiconductor). However, an expensive and large-sized apparatus is indispensable for highly accurate etching.
Conventionally, ion implantation type and combination of substrates aiming at reflectivity control utilizing an ion implantation technique are not taken into consideration, and no report exists of an optical signal processing element utilizing the same. The ion implantation is used mainly to control the electric transmissivity of a semiconductor, and only a research report exists of a current narrowing structure by H+ ion implantation in production of optical elements.
Conventionally, it takes a long time for patterning of minute processing. For example, it takes approximately 10 hours to trace at an accuracy of 0.01 microns in an area of 1 cm square with an electron beam tracing device. For this reason, an element provided by the electron beam exposure/tracing method generally becomes expensive and special.